1. Field of the Invention
The present invention relates to a motor driving apparatus for driving a feed axis and a main axis of a machine tool or arms and the like of an industrial machine and an industrial robot.
2. Description of the Related Art
A motor driving apparatus for driving an alternating-current (AC) motor as a driving source of a feed axis and a main axis of a machine tool or arms and the like of an industrial machine and an industrial robot once converts an AC voltage input from an AC power source side into a direct-current (DC) voltage, further converts into the AC voltage, and then supplies the alternating current to the AC motor. Thus, the motor driving apparatus includes a rectifier which rectifies the AC voltage supplied from the AC power source side and outputs the DC voltage to a DC link (a direct-current link) and an inverter which is connected to the DC link on the AC side of the rectifier and converts the DC voltage of the DC link side into the AC voltage by a switching operation of an internal switching device and supplies the alternating current to the AC motor.
FIG. 5 is a circuit diagram illustrating a general motor driving apparatus which drives a three-phase AC motor using a DC power source. A motor driving apparatus 100 for driving a three-phase AC motor (hereinafter, simply referred to as an “AC motor”) 200 is provided with an inverter 50, and the inverter 50 is applied with a DC voltage from the DC power source is applied to the DC link side of the inverter 50, which outputs a three-phase alternating current for driving the motor 200. Although not illustrated, a rectifier which converts an alternating current input from a commercial AC power source into a direct current and outputs the direct current is generally provided on its DC link side of the inverter 50.
The motor driving apparatus 100 includes the inverter 50, a gate driving circuit 61, a gate driving command generation unit 62, an overcurrent detection unit 63, and a current command generation unit 64. The inverter 50 is constituted of a switching device S and a bridge circuit of a switch unit including a diode D which is connected in reversely parallel to the switching device S, and when the switching device S is turned in/off, the inverter 50 converts the DC voltage on the DC link side into the AC voltage and outputs the AC voltage to the AC motor 200 side. A motor control unit 60 is constituted of the gate driving command generation unit 62, the overcurrent detection unit 63, and the current command generation unit 64. The current command generation unit 64 generates a current command based on an alternating current flowing into the AC motor 200 detected by a motor current detector 71. The gate driving command generation unit 62 outputs either one of an ON command and an OFF command as a gate driving command to the gate driving circuit 61. The gate driving circuit 61 turns on/off on the switching device S of a motor driving unit in response to the received gate driving command. Note that, for simplifying the drawing, only one phase of the gate driving circuit 61 is illustrated. The overcurrent detection unit 63 detects generation of an overcurrent with respect to a current flowing through a DC link detected by a DC link current detector 72 or the alternating current on the AC motor 200 side detected by the motor current detector 71.
As illustrated in FIG. 5, when an abnormal short circuit occurs between output phases on the AC motor 200 side of the motor driving apparatus 100, an overcurrent flowing through a path illustrated in a bold-faced arrow is generated. When the overcurrent continuously flows, each device such as the switching device S breaks down, so that it is needed to interrupt the overcurrent to protect each device. Thus, when the overcurrent detection unit 63 detects the generation of the overcurrent, the gate driving command generation unit 62 generates the OFF command for turning off the switching device S for the gate driving circuit 61, and in response to the command, the gate driving circuit 61 immediately turns off the switching device S and interrupts the overcurrent.
However, since a very large overcurrent is quickly interrupted, a temporal change of the current is large, and a surge voltage caused by an inductance of a current path becomes very large which may be a cause of failure of each device such as the switching device S.
Thus, a snubber circuit 81 for absorbing a surge as illustrated in FIG. 5 is often provided to suppress the surge voltage generated at the time of the overcurrent interruption. In FIG. 5, only one phase of the snubber circuit 81 is illustrated for simplifying the drawing,
In addition to the above, as a method for suppressing the surge voltage generated at the time of the overcurrent interruption, for example, there is a method for reducing a switching speed of the switching device by increasing a gate resistance of the switching device and gently interrupting a current.
As a method for differentiating a switching speed at the time of the overcurrent and that of a normal time, for example, two types of gate resistances, i.e., gate resistances having a large resistance value and having a small one are prepared for the switching device, and the small gate resistance is used in the normal time so as not to reduce the switching speed, and the large gate resistance is used at the time of the overcurrent to reduce the switching speed.
Further, for example, as described in Japanese Patent No. 3692740, there is a method for suppressing a surge voltage by generating a gate voltage pattern including two patterns, i.e., a part in which a gate voltage change is gentle so as to reduce the surge voltage and a part in which the gate voltage change is sharp so as not to increase a switching loss using a gate voltage pattern generator in the normal switching.
As described above, suppression of the surge voltage generated at the time of the overcurrent interruption is important to prevent failure of each device such as the switching device S.
Regarding the method using the snubber circuit in the methods for suppressing the surge voltage generated at the time of the overcurrent interruption, the size of components constituting the snubber circuit will be larger as the surge voltage to be suppressed becomes larger, so that there is a problem that an excessive snubber circuit has to be installed in the normal time other than the time of the overcurrent interruption, and that a cost is increased.
The method for reducing the switching speed of the switching device by increasing the gate resistance has a problem of inefficiency because the switching loss is increased in the normal time in which a faster switching speed causes no problem.
The method for using a plurality of gate resistances having different resistance values has a problem that components of the circuits are increased which increase the cost and deteriorate reliability.
According to the invention described in Japanese Patent No. 3692740, a gate voltage pattern including a part in which the switching speed is faster and a part in which the switching speed is slower has to be generated in one turn-off operation, and there are problems that control is complicated, and components of the circuits will be increased.